FIELD OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a screw type vacuum pump, and, in particular, to an oil-free screw type vacuum pump suited for use in a device treating a process gas that generates reaction products in the pump during a manufacturing of semiconductors.
In, for example, Japanese Patent Unexamined Publication No. 60-216089 and corresponding U.S. Pat. No. (4,714,418) a screw type vacuum pump is proposed which is capable by itself of performing evacuation so as to achieve a low pressure of a level about 10.sup.-4 Torr. The pump is characterised in that working chambers thereof, defined by a male rotor, a female rotor and a casing, include two or three sealed sections between a suction port thereof and a discharge port thereof. The pump includes a working chamber for contributing a transfer stroke, which has been needed by conventional compressors. As the rotors of the pump rotate, the working chambers thereof contribute to the strokes of suction, transfer, compression and discharge, respectively.
When used as a vacuum pump for general gases such as air or nitrogen gas, the above-described conventional pump has no problems. However, when used in a nitride film producing process in a low pressure chemical vapor deposition (CVD) device for manufacturing semiconductors, the rotors of such pump can become locked, which may incapacitate the pump. This is attributable to the great amount of reaction products present on a discharge side of the rotors, in particular on surfaces of tooth spaces contributing to the compression and the discharge strokes and on casing wall surfaces which correspond to the tooth space surfaces.